1. Field of the Invention
The present invention relates a method of separating rhodium from platinum and/or palladium of raw material including rhodium and at least platinum and/or palladium.
2. Description of the Related Art
Platinum group metal such as rhodium, platinum or palladium is included in a residue collected by distilling selenium from copper-electrolyzed deposition (hereinafter referred to as selenium-distilled residue) or a residue obtained by treating scrap such as an automotive exhaust catalyst including platinum group metal such as rhodium, platinum or palladium (hereinafter referred to as rhodium-treated slag). Japanese Patent Application Publication No. 2004-332041 discloses a method including steps of adding sodium bromate to a solution including platinum group metal, oxidizing ruthenium to RuO4, separating RuO4 by distillation, hydrochloridizing the solution, separating palladium by a solvent extraction with di-n-hexyl sulfide (DHS) and separating platinum and iridium by a solvent extraction with tributylphosphate (IBP) in order, as a method of refining and collecting rhodium, platinum and palladium from the solution.
The platinum group metal such as rhodium, platinum or palladium is poorly soluble in a mineral acid under a normal condition. Aqua regia or a mixed liquid of strong oxidant and hydrochloric acid may dissolve platinum and palladium but may not dissolve rhodium. When a material including rhodium, platinum and palladium is dissolved in the aqua regia or the mixed liquid of strong oxidant and hydrochloric acid, the platinum and the palladium exposed in the acid may be dissolved but the rhodium may not be dissolved into the acid. Therefore, the platinum and the palladium covered with the undissolved rhodium may not be dissolved. It is therefore difficult to effectively collect rhodium, platinum and palladium from the mixed material including rhodium, platinum and palladium.
Japanese Patent Application Publication No. 2003-268457 discloses a method including steps of adding sodium hydrate and sodium nitrate to a solution including platinum group metal, leaching selenium and tellurium with water after melting, adding hydrogen peroxide and hydrochloride acid to a residue including platinum group metal, and dissolving the platinum group metal, as a method of dissolving the platinum group metal such as selenium, tellurium, rhodium, platinum or palladium. Japanese Patent Application Publication No. 2005-240170 discloses a method including steps of removing selenium and tellurium by chloridizing and vaporizing in a chlorine gas atmosphere, adding sodium chloride, obtaining soluble salt by chloridizing and roasting the platinum group metal, and dissolving the platinum group metal by water-leaching, as the method of dissolving the platinum group metal such as selenium, tellurium, rhodium, platinum or palladium. However, in the methods, rhodium, platinum and palladium are mixed in an aqueous solution. Solvent extraction of highly concentrated rhodium, platinum and palladium may cause mutual contamination. This may degrade separation efficiency. Therefore, repetition of the solvent extraction causes high cost. It is therefore preferable that rhodium, platinum and palladium are separated from each other with a simple method before dissolving these metals into an aqueous solution in order to collect these metals efficiently.